There are a variety of tools that will do the job ... provided it isn't in too tight which is a very distinct probability. Of course, they were all designed to do other jobs, but there are tools out there. You probably won't find them in Bunnies though.

Unless you want it done in a hurry (in which case the lbs is the best bet), haunt the second hand tools markets and I'm sure you'll find something sooner or later

If you want to make something, go into a model aeroplane shop and buy some spring steel. Modellers use it for undercarriages and it'll probably be the same stuff as Park Tools use to make theirs. Ordinary fencing wire won't do it if you need a lot of force. Just make sure you get exactly the right piece of wire (use the blind end of a drill bit to measure it) because it comes in a wide range of sizes and you won't get it right by guessing.

You'll need a bigarse bolt, two nuts and some penetrene or similar. The bolt has to fit thru the spindle hole in the cup, the nuts have to be a tight fit inside the cup. Spin a nut on far enough so there is enough thread showing to get the second on once you've put the bolt through the hole from the inside. Sandwich the cup between the nuts and tighten down hard. Too see if it will work for you, try turning the bolt head clockwise.

I haven't done this myself, but I've seen it done. Good luck and don't forget the penetrene.
You could still try the LBS...

Shaun

...whatever the road rules, self-preservation is the absolute priority for a cyclist when mixing it with motorised traffic.London Boy 29/12/2011

The left side, the one with the lock ring, is supposed to be loose once the lockring is loosened, so that it can be adjusted.
The drive side cup is the fixed one & is left hand threaded, so will need to be turned CLOCKWISE to LOOSEN it.

The correct tool can be determined once both cranks are removed & the B/B axle taken out. Some of these cups have flats, some don't & have pin holes only.
Strip the rest of it & have a good look. If you don't have a tool that will do the job, then completely strip the rest of the frame & take it to the nearest LBS that has a halfway decent workshop.